(1) Field of the Invention
The present invention relates to novel silica having desirable properties such as high heat-resistance and high water-resistance.
(2) Description of Related Art
Silica has been used as a desiccant for a long time. In recent years, silica has also found its utility as a catalyst carrier, a separating agent, an adsorbent and the like, upon which various different features are demanded for silica. Features of silica depend on various properties such as its specific surface area, pore diameter, pore volume, pore diameter distribution, etc. Such properties are greatly affected by conditions under which silica is produced.
“Silica” means both silicic acid anhydride and silicic acid hydrate. Examples of silicic acid anhydride include quartz, tridymite, cristobalite, coesite, stishovite, quartz glass, etc., while examples of silicic acid hydrate include the so-called amorphous “silica gel”, which is obtained by gelating silica hydrosol and drying the resultant hydrogel. The latter examples also include colloidal silica, silicate oligomer, and silica of the type which is formed using an organic compound or the like as a template (the so-called micelle template type silica), for example, MCM-41 Exxon Mobil Corporation. “Silica gel” can be made from raw materials such as water glass and alkoxysilane.
According to a general method, silica gel is produced by hydrolysis of alkali silicate, such as sodium silicate, with mineral acid followed by gelation and drying of the resultant silica hydrosol. Many suggestions have been made relating to the above production method in order to improve features of silica gel product.
For example, Japanese Patent Laid-Open publication No. SHO 62-113713 discloses a method for producing silica gel with a narrow pore diameter distribution. In this method, alkali silicate solution is hydrolyzed with mineral acid to form silica hydrosol. The hydrosol is gelated and the resultant hydrogel is treated with acid solution whose pH value is 2.5 or less. Then the hydrogel is washed with water and treated with buffer solution so that the pH value of the hydrogel settles within 4–9, and then undergoes hydrothermal treatment.
Japanese Patent Laid-Open publication No. HEI 9-30809 refers to a method of batch fluidized drying of silica hydrogel followed by hydrothermal treatment.
These production methods succeeded in modifying capability of silica gel product to some extent: the pore diameter distribution became sharper. Still, they failed in improving other properties such as its specific surface area, each diameter and the total volume of its pores significantly, and also failed in providing the silica gel product having desirable properties because of its poor heat-resistance and water-resistance.
Additionally, according to the method described in the former publication (SHO 62-113713), the resultant silica generally contains a considerable amount of impurities, such as sodium, magnesium, titanium, aluminum, zirconium, etc., which are derived from alkali silicate being raw material. Even a minute total amount (e.g., few hundred ppm) of such impurities remarkably affects on properties of silica product, for example, in the following three ways: (1) such impurities accelerates crystallization of silica gel at high temperature; (2) such impurities also promote hydrothermal treatment of silica gel with water, resulting in enlargement of the pore diameter, the pore volume and the pore diameter distribution and in reduction in the specific surface area of the silica gel product; and (3) since the presence of such impurities lowers the sintering temperature, calcination of silica gel containing such impurities easily reduces the specific surface area of the silica gel. In particular, alkali metal impurities and alkaline-earth metal impurities cause these effects more readily than other impurities. Besides, silica gel including titanium or aluminum impurities at the silica surface or among siloxane bonds has an increased number of acid sites, causing undesired catalysis when used as a catalyst carrier or an adsorbent.
Examples of the method for producing pure silica gel with little contents of impurities include a method of purifying gel obtained by neutralization of alkali silicate and a method using silicon alkoxide as a raw material. Especially the latter method can give very pure silica gel with less difficulty by purifying the raw material, silicon alkoxide, using technique such as distillation.
The method using silicon alkoxide as a raw material usually includes (i) hydrolysis and condensation process, in which the silicon alkoxide is hydrolyzed into silica hydrosol in the presence of a catalyst and the silica hydrosol is condensed to form silica hydrogel, and (ii) property control process, in which the resultant silica hydrogel is transformed through hydrothermal treatment into silica gel with controlled properties. In (i) hydrolysis and condensation process, acid (sulfuric acid, hydrochloric acid, or nitric acid) is generally used as the catalyst. Additionally, between (i) hydrolysis and condensation process and (ii) property control process, aging process is commonly carried out to intend the improvement of properties (e.g., the hardness) of silica gel product. The above method is called sol-gel method, which is known to the art.
However, silica gel made from silicon alkoxide through sol-gel method generally has a small mean pore diameter and a wide pore diameter distribution. In addition, it is not reported that hydrothermal treatment of silica gel prepared by sol-gel method improves properties of the silica gel significantly.
By the way, Kim et al. (Ultrastable Mesostructured Silica Vesicles Science, 282, 1302 (1998)) disclosed the method of producing a mesoporous molecular sieve with heat-resistance (up to 1000° C.) and water-resistance (at 100° C. for longer than 150 hours) by forming supramolecular structure of electroneutral (non-charged) Gemini surfactant and silica precursor bound with hydrogen bonds and then removing the Gemini surfactants from the supramolecular structure. Kim's method, being a kind of micelle-templated silica technique which forms porous silica product using an organic template, can produce silica with a sharper pore diameter distribution than that produced by the foregoing other methods. However, Kim's method gives inadequate water-resistance to the produced silica, and has complicated processes that result in low productivity.
Macrostructure of silica is well known to the art as continuous three-dimensional structure of spherical particles of silica colloid tightly knitted together, whereas microstructure of silica gel has not yet been fully elucidated. It is therefore expected that analysis of such microstructure of silica lead to finding of a new kind of silica, that fulfills various desired properties with sufficient balance, out of various kinds of silica with previously unknown microstructure.